Aqueous methionine nitrile sulfate



Patented Aug. 14, 1951 UNITED STATES OFFICE AQUEOUS ME'IHIONI NE N ITR ILE: SULFATE .WiHiam'E'Gresham and Carl E. SchweitzeniWilmington, Del.,tas'signors to E. I. 'du Pont de N emours & Company, Wilmington; 'Del., acorporation' ofDelaware No. Drawing. .Application'August3,. 1949,

lSerlal No. 108,421

This invention relates to improvements in'the synthesis of methionine, and to novel compositions of matter employed in the said synthesis. More particularly this application relates to 'alpha-amino-me'thylmercaptobutyronitrile. This application is a continuation-impartof our copending applications. N. 624,328 filed October .24., 1945, now U. S.Patent 2,l85,236, WhlChiIl turn matter. Other objects of this inventio11"will'appear hereinafter.

These and other objects'areaccomplished in accordance with this invention by means ofa series of coacting and interdependent operations comprising the following steps: (1) reaction be tween methyl mercaptan and acroleintoiform beta methylmercaptopropionaldehyde in "the presence of analkaline or'non-acidicfpreferably an amine) catalyst; (Zyaddition of'hydrogen c'y anide to the resultant beta-methylmercaptoproe pionaldehyde, preferably also in .thepresenceof an alkaline catalyst, such" as pyridine, to form alpha hydroxy-gamma-methylmercaptobutyronitrile; (3) reaction of. alpha-hydroxy-gamma methylmercaptobutyronitrile with excess amto.

monia under high pressure to produce alphaaniino-gamrna-methylmercaptobutyronitrile; and (4.) reaction of the said alpha-amino-gammamthylmercaptobutyronitrile with aqueous sulfuric acid, which produces alpha-amino-gammamethylmercaptobutyronitrile sulfate. .On boiling this material with 40-60% aqueous sulfuriciacid, methionine sulfate is produced. Lower or higher concentrations of sulfuric acid result in decreased is usually desirable.

nearly quantitative yield, andthe overallyield of methionine is about 7 The'first step in the synthesis, namely, the reaction between methyl merca-ptan; and acrolein,

is best 'performedat' a temperature of about 0 to C., in' the presence of 0.05 to ,5.0%,;prei,erably 0.1 to 0.5%, of pyridine," basedon the weight-of the methyl mercaptan used. As thereaction'pro- *ceeds, the temperature-may beallowed to increase 10 somewhat, but it shouldnot'be allowed'to-rise above about50- 0., else excessive polymerization of the acrolein will occur.

The methyl mercaptan preferably shouldbe present in excess of the amount theoretically required. In fact, about 2' mols of-methyl'm'ercaptan per mole ofacrolein One reason for this is to permit controlof' the reaction temperature; since the exothermic heat ofthe reaction can-bedissipated conveniently as heat of vaporization'of methyl tmercaptan. Accordingly, the reaction vessel is -generally"equipped with'a reflux con- ;"denser operated at atemperature of 0 C. or lower for return ofvaporized mercaptan to the reaction.

'If desired,theheat-may-beWithdrawn by external cooling. In carryingmut the reaction between acrolein-and'methyl mercaptan; care must be exr ised to pr vent -excessive'polymerization of the acrolein before ithas had; a chance toreact with "thegmercaptan. This is achieved by inhibiting the acrolein with about 0.1'%-ofhydroquinone, and-by very eflicient stirring of;the---mercaptan while addingthe acrolein thereto. 'It is important that the acrolein be, introduced directly into the mercaptanand'not;-for example; alongthe-inner Walls of the reactionyess'el,where-polymerization of the acroleinmay occur because of the alkaline catalys't present. "Towards the end o-f-the reaction,

'whichiusuallyrequires from 10 to minutes, the

"temperature is" preferably allowed to reach about 20 C. After thereaction is complete, the-excess methyl mercaptan' maybe strippedofi, and, if desired, the residue maybe distilled to yield as the almost exclusive product; beta-methylmercaptopropionaldehyde. This novel procedure gives beta methylmercaptopropionaldehydein very high yield ('98 70).

The next step 'in theprocess'ofthis invention I is the preparation of the-cyanohydrin ofbetamethylmercaptopropionaldehyde, namely, alphayield due to the formation of tars. Similarly/"hy 0 y amm -m thy m cap 0 yr0ni r 1 phosphoric acid in place of sulfuric acid produces tars almost to the exclusion of m'ethiohinelphos phate. In accordance with this invention, .this synthesis of alpha-amino,-gammaem'ethylmerwhich "is 5 prepared also bythe use of-pyridine' as the 5 preferred catalyst. Consequently, the betamethyl-mercaptopropionaldehyde obtained in the preceding step need '--not necessari1ybe distilled captobutyronitrile sulfate is accomplished "in '65or--otherwise"freed 'of pyridine, henceacrolein methyl mercaptan addition product, containing pyridine, may be treated directly with hydrogen cyanide. In the second step of this invention, hydrogen cyanide, preferably in about 10% excess, based upon the beta-methyl-mercaptopropionaldehyde is added to the reaction product of the first step at a temperature in the range of about 15 to 75 C. (preferably 35 to 55 C.) with 0.1 to 0.5% of an amine catalyst, such as pyridine. The formation of the cyanhydrin takes place very easily, yield after about 30 minutes reaction time being practically quantitative.

In a specific embodiment, this hydroxynitrile, namely alpha-hydroxy-gamma-methylmercaptobutyronitrile, may be isolated in pure form, and thereafter aminated as hereinafter set forth. This alpha hydroxy gamma-methylmercaptobutyronitrile is a novel composition of matter which is particularly useful in, and especially adapted for, the herein described process for making methionine. Prior investigators in this field have never succeeded in isolating this material, or even obtaining it in crude or transitory form, and, consequently, these prior processes for making methionine from beta-methylmercaptopropionaldehyde without obtaining this inter mediate have invariably given low yields.

The hydroxynitrile mentioned above has outstanding utility as a methionine precursor or intermediate. The conversion of the hydroxynitrile to methionine is accomplished by a series of steps, the first of which is to subject it to the action of ammonia. It is not necessary to separate the pyridine or other amine catalyst from the product of the preceding step prior to carrying on this amination, but such separation may be effected if desired. The reaction of alphahydroxy gamma methylmercaptobutyronitrile with ammonia is preferably accomplished by processing alpha hydroxy-gamma-methylmercaptobutyronitrile at a temperature in the range of 10 to 150 C., preferably 75 to 90 C., with a large excess of ammonia (at least 5 moles of ammonia per mole of cyanhydrin, preferably to 30 moles), at a superatmospheric pressure of 5 to 1000 atmospheres, preferably about 10 to 100 atmospheres. At about room temperature the reaction may require up to 12 to 15 hours or more for completion, but at 80 to 90 C., the reaction is complete in 15 to minutes. The yield of alpha amino gamma methylmercaptobutyronitrile (i. e., methionine nitrile) under these conditions is virtually quantitative.

This aminonitrile (i. e., methionine nitrile) is disclosed in our earlier applications S. N. 522,966 and S. N. 624,328. Prior attempts to obtain either this aminonitrile or the hydrochloride thereof have invariably met with failure. For example, Barger and Coyne (loc. cit., pages 1420-1) reported that attempts to obtain the hydrochloride of this aminonitrile have not been successful. The success of the present methionine synthesis is due, in large measure, to the fact that the hydroxynitrile, and the aminonitrile, are both obtained in isolable form. The pure aminonitrile, which is described in detail below, is a thermally unstable compound, which cannot be distilled under ordinary conditions, i. e. at pressures above a few millimeters. It can be hydrolyzed, however, to give methionine in good yields. Specific methods for the hydrolysis of this nitrile are set forth in U. S. Patents 2,432,478, 2,443,391 and 2,432,429. The successful preparation of the aminonitrile is probably due to the fact that the cyanhydrin is first obtainable, as stated above,

in isolable form, and is thereafter treated with high pressure ammonia. Ordinary ammoniation does not give a satisfactory result. In fact, unless very large excess, or high pressure, of ammonia is used, preferably above about 30 atmospheres, a mixed product containing appreciable amounts of the unreacted cyanhydrin is obtained. The purest aminonitrile is thus prepared at high ammoniation pressures. This is important since the separation of the aminonitrile from the cyanhydrin by physical methods is quite difficult or virtually impossible. This is due in part to the thermal instability of the aminonitrile.

Because of the almost quantitative yield in the amination step, the hydrolysis of alpha-aminogamma-methylmercaptobutyronitrile to methionine may be effected without intermediate steps or processing of materials. In continuous operation, the product of the preceding step is pumped through a reaction vessel along with hot aqueous acid, such as sulfuric acid of about 10 to 10% concentration, preferably about 50%. This produces an aqueous solution of methionine nitrile sulfate. A preferred procedure is to mix the alpha amino gamma methylmercaptobutyronitrile with 50% sulfuric acid, and to heat the resulting methionine nitrile sulfate solution at a temperature close to the boiling point for about an hour or longer. Thereafter the mixture is cooled to about room temperature, and the sulfuric acid is neutralized with ammonia. In certain instances the product at this stage has a rather dark color, which can be removed by means of decolorizing charcoal. To isolate methionine from the resultant solution, any of the conventional techniques for separating a solute from a solvent may be employed. The water may be removed at low pressure, and the methionine can be extracted from the ammonium sulfate in the resulting residue by a suitable solvent. Alternatively, the solvent may be evaporated and the product isolated by fractional crystallization. The yield in the hydrolysis step generally is within the range of 70 to 75%, calculated on the basis of the pure recrystallized product.

Example-Into a 200 cubic centimeter 3- necked flask equipped with a stirrer, a Dry-Ice reflux condenser, a thermometer and a dropping funnel is placed a mixture of 96 grams of methyl mercaptan and 0.3 gram of pyridine. With rapid stirring, acrolein (56 grams) containing 0.1% hydroquinone is introduced dropwise through the dropping funnel, initial temperature being 5 C. As the reaction proceeds it is necessary to absorb part of the reaction heat by surrounding the reaction vessel with an ice water bath. The reaction continues for about one hour, during which time the reaction mixture is allowed to warm up to 20 C. The unreacting mercaptan (52 grams) and acrolein (3.6 grams) are recovered by distillation at low pressure, the receiver being a cold trap at C. There remains a residue which distills quite completely at 60 C., 12 mm., which is the boiling point of beta-methylmercaptopropionaldehyde (weight of betamethyl-mercaptopropionaldehyde, 96 grams). This aldehyde is returned to the reaction vessel and a little pyridine (0.3 gram) is added. Into the mixture is introduced 27.2 grams of liquid hydrogen cyanide. By suitable cooling, the temperature of the mixture is maintained at 40 to 45 C. for about 20 minutes. The formation of the cyanhydrin takes place quite smoothly. When the reaction is complete the excess hydrogen cyanide is removed by low pressure distillation into a cold trap. There remains 118 grams of the cyanhydrin, i. e., alpha-hydroxy-gammamethylmercaptobutyronitrile.

If it is desired to convert the cyanhydrin to methionine, the product thus obtained is transferred to a pressure resistant silver-lined vessel of about 900 cc. capacity, and ammonia (300 grams) is injected. The vessel is then agitated and heated to a temperature of 80 to 90 C., the contents being under an autogenously developed pressure or" about 40 atmospheres. After 15 minutes at this temperature the reaction mixture is cooled, and then withdrawn from the reaction vessel. Upon evaporation of the ammonia there remains a residue which is a mixture of water and alpha-amino-gamma-methylmercaptobutyronitrile. An aliquot portion of this residue is placed in a distilling flask and water is removed therefrom by evaporation at low pressure. The remaining residue, which is alphaamino-gamma methylmercaptobutyronitrile, is obtained in quantity corresponding to nearly quantitative yield. This residue is thereafter added to the main portion of the aqueous aminonitrile. Without separating the water from this aminonitrile, the mixture is added dropwise to a solution containing 180 grams of C017:- oentrated sulfuric acid and 189 grams of water. The resulting mixture, which is an aqueous solution of the sulfuric acid salt of methionine nitrile, is boiled for 1.5 hours. Thereafter, it is cooled to room temperature, and neutralized with aqueous ammonium hydroxide. The solution thus obtained is decolorized by boiling with a few grams of decolorizing charcoal. After removal of the charcoal by filtration, the solution is placed in an evaporator, and water is removed at diminished pressure until there remains a solid mixture of methionine and ammonum sulfate. By repeated extraction with liquid ammonia, the methionine is removed from the ammonium sulfate. The methionine is isolated by evaporation of ammonia from this extract. There is obtained crude methionine which on recrystallization from water yields 112 grams of pure methionine, melting at 283 C.

It will be understood that the present invention is not limited to the particular embodiments which are hereinbefore used as illustrations. For example, it is not essential that the same catalyst, pyridine, be employed in the first two steps in 6 the synthesis of methionine, although generally there is an advantage in doing so. If desired, other non-acidic catalysts may be employed for the reaction between acrolein and methyl mercaptan. Thus, charcoal, amines (piperidine, quinoline, methionine nitrile, triethanolamine, etc), lime, alkali metal alcoholates and the like are operative as catalysts in the first step. If desired, the product obtained in step (2) (alphahydroxy-gamma methylmercaptobutyronitrile) may be neutralized or acidified prior to the amination, because its storage properties are better in the absence of alkali. Other acids besides sulfuric acid may be employed in the hydrolysis of the aminonitrile to methionine, although comparatively poor results are obtained with hydrochloric acid. For example, when equimolal quantities of the aminonitrile and 37% hydrochloric acid are heated at refluxing temperature for one hour, or allowed to stand overnight at room temperature, a poor conversion (less than 29%) to methionine is obtained. So far as methionine synthesis is concerned, the improved results reported herein are due in part to the formation of methionine nitrile sulfate prior to hydrolysis.

Since many different embodiments of this invention may be made without departing from the spirit and scope thereof, it will be understood that we do not limit ourselves except as set forth in the appended claim.

We claim:

Aqueous alpha-amino-gamma-methylmercaptobutyronitrile sulfate.

WILLIAM F. GRESHAM. CARL E. SCHWEITZER.

REFERENCES CITED The following references are of record in the file of'this patent:

UNITED STATES PATENTS Number Name Date 2,432,478 Lecky Dec. 9, 1947 2,443,391 Kirkpatrick June 15, 1948 2,435,236 Gresham et al Oct. 18, 1949 OTHER REFERENCES Barger et al., Biochem. J., vol 22 (1928), pp.14201421.

Catch et al., Nature, April 26, 1947, pp. 518- 519. 

